Enanta Pharmaceuticals (ENTA) 10-Q 10 Feb 20 2020 Q1 Quarterly report Financial data

Document and Entity Information

Document and Entity Information - shares	3 Months Ended
	Dec. 31, 2019	Jan. 31, 2020
Cover [Abstract]
Document Type	10-Q
Amendment Flag	false
Document Period End Date	Dec. 31, 2019
Document Fiscal Year Focus	2020
Document Fiscal Period Focus	Q1
Trading Symbol	ENTA
Entity Registrant Name	ENANTA PHARMACEUTICALS, INC.
Entity Central Index Key	0001177648
Current Fiscal Year End Date	--09-30
Entity Filer Category	Large Accelerated Filer
Entity Small Business	false
Entity Emerging Growth Company	false
Entity Shell Company	false
Entity File Number	001-35839
Security Exchange Name	NASDAQ
Entity Tax Identification Number	04-3205099
Entity Address, Address Line One	500 Arsenal Street
Entity Address, City or Town	Watertown
Entity Address, State or Province	MA
Entity Address, Postal Zip Code	02472
City Area Code	617
Title of 12(b) Security	Common Stock, par value $0.01 per share
Local Phone Number	607-0800
Entity Incorporation, State or Country Code	DE
Document Transition Report	false
Document Quarterly Report	true
Entity Interactive Data Current	Yes
Entity Current Reporting Status	Yes
Entity Common Stock, Shares Outstanding		19,924,426

Consolidated Balance Sheets

Consolidated Balance Sheets - USD ($) $ in Thousands	Dec. 31, 2019	Sep. 30, 2019
Current assets:
Cash and cash equivalents	$ 27,558	$ 51,230
Short-term marketable securities	346,227	284,006
Accounts receivable	52,570	51,313
Prepaid expenses and other current assets	14,153	15,299
Total current assets	440,508	401,848
Long-term marketable securities	40,941	65,013
Property and equipment, net	10,407	10,927
Deferred tax assets	10,656	11,341
Operating lease, right-of-use assets	7,762
Restricted cash	608	608
Other long-term assets	92	92
Total assets	510,974	489,829
Current liabilities:
Accounts payable	6,596	6,689
Accrued expenses and other current liabilities	9,411	15,920
Operating lease liabilities	3,132
Total current liabilities	19,139	22,609
Operating lease liabilities, net of current portion	5,987
Series 1 nonconvertible preferred stock	1,628	1,628
Other long-term liabilities	1,933	3,100
Total liabilities	28,687	27,337
Commitments and contingencies (Note 12)
Stockholders' equity:
Common stock; $0.01 par value per share, 100,000 shares authorized; 19,810 and 19,703 shares issued and outstanding at December 31, 2019 and September 30, 2019, respectively	198	197
Additional paid-in capital	304,672	298,409
Accumulated other comprehensive income	234	146
Retained earnings	177,183	163,740
Total stockholders' equity	482,287	462,492
Total liabilities and stockholders' equity	$ 510,974	$ 489,829

Consolidated Balance Sheets (Pa

Consolidated Balance Sheets (Parenthetical) - $ / shares	Dec. 31, 2019	Sep. 30, 2019
Statement Of Financial Position [Abstract]
Common stock, par value	$ 0.01	$ 0.01
Common stock, shares authorized	100,000,000	100,000,000
Common stock, shares issued	19,810,000	19,703,000
Common stock, shares outstanding	19,810,000	19,703,000

Consolidated Statements of Oper

Consolidated Statements of Operations - USD ($) shares in Thousands, $ in Thousands	3 Months Ended
	Dec. 31, 2019	Dec. 31, 2018
Income Statement [Abstract]
Royalty revenue	$ 52,570	$ 69,886
Type Of Revenue Extensible List	us-gaap:RoyaltyMember	us-gaap:RoyaltyMember
Operating expenses:
Research and development	$ 32,778	$ 34,878
General and administrative	6,921	7,152
Total operating expenses	39,699	42,030
Income from operations	12,871	27,856
Other income (expense):
Interest income (expense), net	2,076	1,885
Total other income (expense), net	2,076	1,885
Income before income taxes	14,947	29,741
Income tax expense	(1,504)	(3,730)
Net income	$ 13,443	$ 26,011
Net income per share:
Basic	$ 0.68	$ 1.34
Diluted	$ 0.65	$ 1.25
Weighted average shares outstanding:
Basic	19,751	19,426
Diluted	20,773	20,810

Consolidated Statements of Comp

Consolidated Statements of Comprehensive Income - USD ($) $ in Thousands	3 Months Ended
	Dec. 31, 2019	Dec. 31, 2018
Statement Of Income And Comprehensive Income [Abstract]
Net income	$ 13,443	$ 26,011
Other comprehensive income:
Net unrealized gains on marketable securities, net of tax of ($28) and ($45)	88	141
Total other comprehensive income, net of tax	88	141
Comprehensive income	$ 13,531	$ 26,152

Consolidated Statements of Co_2

Consolidated Statements of Comprehensive Income (Parenthetical) - USD ($) $ in Thousands	3 Months Ended
	Dec. 31, 2019	Dec. 31, 2018
Statement Of Income And Comprehensive Income [Abstract]
Net unrealized gains (losses) on marketable securities, tax	$ (28)	$ (45)

Consolidated Statements of Stoc

Consolidated Statements of Stockholders' Equity - USD ($) shares in Thousands, $ in Thousands	Total	Common Stock [Member]	Additional Paid-In Capital [Member]	Accumulated Other Comprehensive Income (Loss) [Member]	Retained Earnings [Member]
Beginning Balance at Sep. 30, 2018	$ 393,679	$ 194	$ 276,526	$ (398)	$ 117,357
Beginning Balance, Shares at Sep. 30, 2018		19,395
Exercise of stock options	1,161		1,161
Exercise of stock options, Shares		40
Stock-based compensation expense	5,843		5,843
Total other comprehensive income, net of tax	141			141
Net income	26,011				26,011
Ending Balance at Dec. 31, 2018	426,835	$ 194	283,530	(257)	143,368
Ending Balance, Shares at Dec. 31, 2018		19,435
Beginning Balance at Sep. 30, 2019	462,492	$ 197	298,409	146	163,740
Beginning Balance, Shares at Sep. 30, 2019		19,703
Exercise of stock options	$ 2,306	$ 1	2,305
Exercise of stock options, Shares	76	76
Vesting of restricted stock units, net of withholding	$ (1,140)		(1,140)
Vesting of restricted stock units, net of withholding, Shares		31
Stock-based compensation expense	5,098		5,098
Total other comprehensive income, net of tax	88			88
Net income	13,443				13,443
Ending Balance at Dec. 31, 2019	$ 482,287	$ 198	$ 304,672	$ 234	$ 177,183
Ending Balance, Shares at Dec. 31, 2019		19,810

Consolidated Statements of Cash

Consolidated Statements of Cash Flows - USD ($) $ in Thousands	3 Months Ended
	Dec. 31, 2019	Dec. 31, 2018
Cash flows from operating activities
Net income	$ 13,443	$ 26,011
Adjustments to reconcile net income to net cash provided by operating activities:
Stock-based compensation expense	5,098	5,843
Depreciation and amortization expense	904	680
Deferred income taxes	657	(918)
Premium paid on marketable securities	(988)
Accretion of discount on marketable securities	(330)	(977)
Other non-cash items	(2)	39
Change in operating assets and liabilities:
Accounts receivable	(1,257)	(2,681)
Prepaid expenses and other current assets	1,146	(3,182)
Operating lease, right-of-use assets	520
Accounts payable	(179)	124
Accrued expenses	(5,989)	598
Income taxes payable		4,470
Operating lease liabilities	(593)
Other long-term liabilities	(67)	247
Net cash provided by operating activities	12,363	30,254
Cash flows from investing activities
Purchase of marketable securities	(117,461)	(119,546)
Proceeds from maturities and sale of marketable securities	80,748	98,999
Purchase of property and equipment	(488)	(384)
Net cash used in investing activities	(37,201)	(20,931)
Cash flows from financing activities
Proceeds from exercise of stock options	2,306	1,161
Payments for settlement of share-based awards	(1,140)
Payments of capital lease obligations		(21)
Net cash provided by financing activities	1,166	1,140
Net increase (decrease) in cash, cash equivalents and restricted cash	(23,672)	10,463
Cash, cash equivalents and restricted cash at beginning of period	51,838	64,510
Cash, cash equivalents and restricted cash at end of period	28,166	74,973
Supplemental disclosure of cash flow information:
Cash paid for income taxes		5
Supplemental disclosure of non-cash investing information:
Purchases of fixed assets included in accounts payable and accrued expenses	216	$ 1,304
Operating lease liabilities arising from obtaining right-of-use assets	$ 1,131

Nature of the Business and Basi

Nature of the Business and Basis of Presentation	3 Months Ended
	Dec. 31, 2019
Organization Consolidation And Presentation Of Financial Statements [Abstract]
Nature of the Business and Basis of Presentation	1. Nature of the Business and Basis of Presentation Enanta Pharmaceuticals, Inc. (the “Company”), incorporated in Delaware in 1995, a biotechnology company that uses its robust, chemistry-driven approach and drug discovery capabilities to create small molecule drugs primarily for the treatment of viral infections and liver diseases. The Company discovered glecaprevir, the second protease inhibitor discovered and developed through its collaboration with AbbVie for the treatment of chronic hepatitis C virus, or HCV. Glecaprevir is co-formulated as part of AbbVie’s leading direct-acting antiviral (DAA) combination treatment for HCV, which is marketed under the tradenames MAVYRET ® Royalties from the Company’s AbbVie collaboration and its existing financial resources provide funding to support the Company’s wholly-owned research and development programs , which are primarily focused on the following disease targets: The Company is subject to many of the risks common to companies in the biotechnology industry including, but not limited to, the uncertainties of research and development, competition from technological innovations of others, dependence on collaborative arrangements, protection of proprietary technology, dependence on key personnel and compliance with government regulation. Product candidates currently under development will require significant additional research and development efforts, including extensive preclinical and clinical testing and regulatory approvals, prior to commercialization. These efforts require significant amounts of capital, adequate personnel infrastructure, and extensive compliance reporting capabilities. Unaudited Interim Financial Information The consolidated balance sheet at September 30, 2019 was derived from audited financial statements, but does not include all disclosures required by accounting principles generally accepted in the United States of America (“GAAP”). The accompanying unaudited consolidated financial statements as of December 31, 2019 and for the three months ended December 31, 2019 and 2018 have been prepared by the Company pursuant to the rules and regulations of the Securities and Exchange Commission (“SEC”) for interim financial statements. Certain information and footnote disclosures normally included in financial statements prepared in accordance with GAAP have been condensed or omitted pursuant to such rules and regulations. These financial statements should be read in conjunction with the Company’s audited financial statements and the notes thereto included in the Company’s Annual Report on Form 10-K for the year ended September 30, 2019. In the opinion of management, all adjustments, consisting of normal recurring adjustments necessary for a fair statement of the Company’s financial position as of December 31, 2019 and results of operations for the three months ended December 31, 2019 and 2018 and cash flows for the three months ended December 31, 2019 and 2018, have been made. The results of operations for the three months ended December 31, 2019 are not necessarily indicative of the results of operations that may be expected for subsequent quarters or the year ending September 30, 2020. The accompanying consolidated financial statements have been prepared in conformity with GAAP. All amounts in the consolidated financial statements and in the notes to the consolidated financial statements, except per share amounts, are in thousands unless otherwise indicated.

Summary of Significant Accounti

Summary of Significant Accounting Policies	3 Months Ended
	Dec. 31, 2019
Accounting Policies [Abstract]
Summary of Significant Accounting Policies	2. Summary of Significant Accounting Policies For the Company’s Significant Accounting Policies, please refer to its Annual Report on Form 10-K for the fiscal year ended September 30, 2019. Any reference in these notes to applicable guidance is meant to refer to the authoritative GAAP as found in the Accounting Standards Codification (“ASC”) and Accounting Standards Update (“ASU”) of the Financial Accounting Standards Board (“FASB”). Use of Estimates The preparation of consolidated financial statements in conformity with GAAP requires management to make estimates and assumptions that affect the reported amounts of assets and liabilities, the disclosure of contingent assets and liabilities at the date of the consolidated financial statements, and the reported amounts of revenues and expenses during the reporting period. Significant estimates and assumptions reflected in these consolidated financial statements include, but are not limited to, management’s judgments with respect to its revenue arrangements; valuation of Series 1 nonconvertible preferred stock and stock-based awards; the accrual of research and development expenses, and the accounting for income taxes, including uncertain tax positions and the valuation of net deferred tax assets. Estimates are periodically reviewed in light of changes in circumstances, facts and experience. Actual results could differ from the Company’s estimates. Recently Adopted Accounting Pronouncements The Company adopted ASU No. 2016-02, Leases (Topic 842), as of October 1, 2019, using the modified retrospective method under ASU No. 2018-11, Leases (Topic 842): Targeted Improvements. The transition method allows entities to apply the transition requirements at the effective date rather than at the beginning of the earliest comparative period presented. The Company’s reporting for comparative periods is presented in accordance with ASC 840, Leases. Adoption of the new standard resulted in the recording of right of use (“ROU”) assets and lease liabilities of $7,151 and $8,622, respectively. The adoption of the standard did not have a material impact on the Company’s results of operations or cash flows. The Company elected to use the transition package of three practical expedients, which among other things, allowed the Company to carry forward the historical lease classification. The Company has elected, under Topic 842, the further practical expedient not to separate non-lease components from the lease components to which they relate and instead to combine them and account for them as a single lease component. The Company also elected the accounting policy election to keep leases with a term of twelve months or less off the balance sheet and to recognize payments for those leases on a straight-line basis over the lease term. ROU assets represent the Company’s right to use an underlying asset for the lease term, and lease liabilities represent the Company’s obligation to make lease payments arising from the lease. At the inception of the arrangement, the Company determines if an arrangement is a lease based on assessment of the terms and conditions of the contract. Operating lease ROU assets and lease liabilities are recognized at commencement date, and thereafter if modified, based on the present value of lease payments over the lease term. The lease term includes any renewal or early-termination options that the Company is reasonably assured to exercise. The present value of lease payments is determined by using the interest rate implicit in the lease, if that rate is readily determinable; otherwise, the Company uses its estimated secured incremental borrowing rate for that lease term. The underlying assets of the Company’s leases as of the adoption date consisted of office and laboratory space. In March 2017, the FASB issued ASU No. 2017-08, Receivables—Nonrefundable Fees and Other Costs (Subtopic 310-20): Premium Amortization on Purchased Callable Debt Securities (“ASU 2017-08”) which requires companies to amend the amortization period for premiums on debt securities with explicit call features to be the period through the earliest call date rather than through the contractual life of the debt instrument. This amendment aims to more closely align the recognition of interest income with the manner in which market participants price such instruments. The Company adopted the new standard on the effective date of October 1, 2019. T financial position and results of operations. Recently Issued Accounting Pronouncements In June 2016, the FASB issued ASU No. 2016-13, Financial Instruments—Credit Losses (Topic 326) (“ASU 2016-13”), which introduces a new methodology for accounting for credit losses on financial instruments, including available-for-sale debt securities. The guidance establishes a new “expected loss model” that requires entities to estimate current expected credit losses on financial instruments by using all practical and relevant information. Any expected credit losses are to be reflected as allowances rather than reductions in the amortized cost of available-for-sale debt securities. This amendment is effective for the Company in the fiscal year beginning October 1, 2020. The Company is currently evaluating the potential impact that ASU 2016-13 may have on its financial position and results of operations. Other accounting standards that have been issued or proposed by the FASB or other standards-setting bodies that do not require adoption until a future date are not expected to have a material impact on the Company’s consolidated financial statements upon adoption.

Fair Value of Financial Assets

Fair Value of Financial Assets and Liabilities	3 Months Ended
	Dec. 31, 2019
Fair Value Disclosures [Abstract]
Fair Value of Financial Assets and Liabilities	3. Fair Value of Financial Assets and Liabilities The following tables present information about the Company’s financial assets and liabilities that were subject to fair value measurement on a recurring basis as of December 31, 2019 and September 30, 2019, and indicate the fair value hierarchy of the valuation inputs utilized to determine such fair value: Fair Value Measurements at December 31, 2019 Using: Level 1 Level 2 Level 3 Total (in thousands) Assets: Cash equivalents: Money market funds $ 6,065 $ — $ — $ 6,065 Commercial paper — 17,423 — 17,423 Marketable securities: U.S. Treasury notes 176,856 — — 176,856 Commercial paper — 77,269 — 77,269 Corporate bonds — 133,043 — 133,043 $ 182,921 $ 227,735 $ — $ 410,656 Liabilities: Series 1 nonconvertible preferred stock $ — $ — $ 1,628 $ 1,628 $ — $ — $ 1,628 $ 1,628 Fair Value Measurements at September 30, 2019 Using: Level 1 Level 2 Level 3 Total (in thousands) Assets: Cash equivalents: Money market funds $ 44,569 $ — $ — $ 44,569 Marketable securities: U.S. Treasury notes 170,515 — — 170,515 Corporate bonds — 111,837 — 111,837 Commercial paper — 66,667 — 66,667 $ 215,084 $ 178,504 $ — $ 393,588 Liabilities: Series 1 nonconvertible preferred stock $ — $ — $ 1,628 $ 1,628 $ — $ — $ 1,628 $ 1,628 During the three months ended December 31, 2019 and 2018, there were no transfers between Level 1, Level 2 and Level 3. The outstanding shares of Series 1 nonconvertible preferred stock are measured at fair value. The fair value of these instruments was based on significant inputs not observable in the market, which represented a Level 3 measurement within the fair value hierarchy. The Company utilized a probability-weighted valuation model which takes into consideration various outcomes that may require the Company to transfer assets upon exercise. Changes in the fair value of the Series 1 nonconvertible preferred stock are recognized in other income (expense), net in the consolidated statements of operations. The recurring Level 3 fair value measurements of the Company’s outstanding Series 1 nonconvertible preferred stock using probability-weighted discounted cash flow include the following significant unobservable inputs: Range (Weighted Average) December 31, September 30, Unobservable Input 2019 2019 Probabilities of payout 0%-60% 0%-60% Discount rate 5.75% 6.00% The following table provides a rollforward of the aggregate fair values of the Company’s outstanding Series 1 nonconvertible preferred stock for which fair value is determined by Level 3 inputs: Series 1 Nonconvertible Preferred Stock Balance, September 30, 2019 $ 1,628 Change in fair value of nonconvertible preferred stock — Balance, December 31, 2019 $ 1,628

Marketable Securities

Marketable Securities	3 Months Ended
	Dec. 31, 2019
Investments Debt And Equity Securities [Abstract]
Marketable Securities	4. Marketable Securities As of December 31, 2019 and September 30, 2019, the fair value of available-for-sale marketable securities, by type of security, was as follows: December 31, 2019 Amortized Cost Gross Unrealized Gains Gross Unrealized Losses Fair Value (in thousands) U.S. Treasury notes $ 176,712 $ 144 $ — 176,856 Corporate bonds 132,928 127 (12 ) 133,043 Commercial Paper 77,269 — — 77,269 $ 386,909 $ 271 $ (12 ) $ 387,168 September 30, 2019 Amortized Cost Gross Unrealized Gains Gross Unrealized Losses Fair Value (in thousands) U.S. Treasury notes $ 170,519 $ 60 $ (64 ) $ 170,515 Corporate bonds 111,690 170 (23 ) 111,837 Commercial Paper 66,667 — — 66,667 $ 348,876 $ 230 $ (87 ) $ 349,019 As of December 31, 2019 and September 30, 2019 marketable securities consisted of investments that mature within one year, with the exception of certain corporate bonds and U.S. Treasury notes, which have maturities between one and three years and an aggregate fair value of $40,941 and $65,013, respectively.

Accrued Expenses and Other Long

Accrued Expenses and Other Long-Term Liabilities	3 Months Ended
	Dec. 31, 2019
Payables And Accruals [Abstract]
Accrued Expenses and Other Long-Term Liabilities	5. Accrued Expenses and Other Long-Term Liabilities Accrued expenses and other current liabilities, as well as other long-term liabilities, consisted of the following as of December 31, 2019 and September 30, 2019: December 31, September 2019 2019 (in thousands) Accrued expenses: Accrued research and development expenses $ 3,877 $ 6,936 Accrued payroll and related expenses 1,628 3,894 Accrued clinical manufacturing 2,580 3,447 Accrued professional fees 921 759 Accrued other 405 884 $ 9,411 $ 15,920 Other long-term liabilities: Uncertain tax positions $ 1,679 $ 1,746 Accrued rent expense — 900 Capital lease obligation — 200 Asset retirement obligation 254 254 $ 1,933 $ 3,100

AbbVie Collaboration

AbbVie Collaboration	3 Months Ended
	Dec. 31, 2019
Organization Consolidation And Presentation Of Financial Statements [Abstract]
AbbVie Collaboration	6. AbbVie Collaboration The Company has a Collaborative Development and License Agreement (as amended, the “AbbVie Agreement”), with AbbVie to identify, develop and commercialize HCV NS3 and NS3/4A protease inhibitor compounds, including paritaprevir and glecaprevir, under which the Company has received license payments, proceeds from a sale of preferred stock, research funding payments, milestone payments and royalties totaling approximately $922,000 through December 31, 2019. Since the Company satisfied all of its performance obligations under the AbbVie Agreement by the end of fiscal 2011, all milestone payments received since then have been recognized as revenue when the milestones were achieved by AbbVie. The Company is receiving annually tiered royalties per Company protease product ranging from ten percent up to twenty percent, or on a blended basis from ten percent up to the high teens, on the portion of AbbVie’s calendar year net sales of each HCV regimen that is allocated to the protease inhibitor in the regimen. Beginning with each January 1, the cumulative net sales of a given royalty-bearing protease inhibitor product start at zero for purposes of calculating the tiered royalties on a product-by-product basis.

Series 1 Nonconvertible Preferr

Series 1 Nonconvertible Preferred Stock	3 Months Ended
	Dec. 31, 2019
Debt Disclosure [Abstract]
Series 1 Nonconvertible Preferred Stock	7. Series 1 Nonconvertible Preferred Stock As of December 31, 2019, 1,931 shares of Series 1 nonconvertible preferred stock were issued and outstanding. Since these shares qualify as a derivative, the outstanding shares are carried at fair value as a liability on the Company’s consolidated balance sheet.

Stock-Based Awards

Stock-Based Awards	3 Months Ended
	Dec. 31, 2019
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]
Stock-Based Awards	8. Stock-Based Awards The Company grants stock-based awards, including stock options and unit awards under its 2019 Equity Incentive Plan (the “2019 Plan”), which was approved by its stockholders on February 28, 2019. The Company also has outstanding unit awards, stock options and restricted stock unit awards under its 2012 Equity Incentive Plan (the “2012 Plan”) and its amended and restated 1995 Equity Incentive Plan (the “1995 Plan”), but is no longer granting awards under these plans. The following table summarizes stock option activity, including performance-based options, for the year-to-date period ending December 31, 2019: Shares Issuable Under Options Weighted Average Exercise Price Weighted Average Remaining Contractual Term Aggregate Intrinsic Value (in thousands) (in years) (in thousands) Outstanding as of September 30, 2019 2,967 $ 46.54 6.7 $ 57,336 Granted 577 63.32 Exercised (76 ) 30.16 Forfeited (23 ) 75.36 Outstanding as of December 31, 2019 3,445 $ 49.53 6.8 $ 58,637 Options vested and expected to vest as of December 31, 2019 3,445 $ 49.53 6.8 $ 58,637 Options exercisable as of December 31, 2019 2,002 $ 38.28 5.4 $ 51,630 Market and Performance-Based Stock Unit Awards The Company awards both performance share units, or PSUs, and relative total stockholder return units, or rTSRUs, to its executive officers. The number of units represents the target number of shares of common stock that may be earned; however, the actual number of shares that may be earned ranges from 0% to 150% of the target number. PSUs rTSRUs Shares Weighted Average Grant Date Fair Value Shares Weighted Average Grant Date Fair Value (in thousands, except per share data) Unvested at September 30, 2019 41 $ 73.02 41 $ 67.76 Granted 27 63.94 27 45.81 Vested — — — — Cancelled — — — — Unvested at December 31, 2019 68 $ 69.37 68 $ 58.94 Restricted Stock Units During the three months ended December 31, 2016, the Company awarded restricted stock units to its employees, which vest 50% in three years and 50% in four years, provided the employee remains employed with the Company at the time of vesting. The fair value per share of these awards was determined based on the intrinsic value of the stock on the date of grant and is being recognized as stock-based compensation expense over the requisite service period. The following table summarizes the restricted stock unit activity for the year-to-date period ending December 31, 2019: Restricted Stock Units Weighted Average Grant Date Fair Value (in thousands, except per share data) Unvested at September 30, 2019 95 $ 30.00 Granted — — Vested (48 ) 30.00 Cancelled (1 ) 30.00 Unvested at December 31, 2019 46 $ 30.00 Stock-Based Compensation Expense During the three months ended December 31, 2019 and 2018, the Company recognized the following stock-based compensation expense: Three Months ended December 31, 2019 2018 (in thousands) Research and development $ 2,492 $ 2,274 General and administrative 2,606 3,569 $ 5,098 $ 5,843 Three Months ended December 31, 2019 2018 (in thousands) Stock options $ 4,335 $ 3,904 Performance stock units 243 1,206 rTSRUs 362 531 Restricted stock units 158 202 $ 5,098 $ 5,843 During the three months ended December 31, 2019 and 2018, the Company recognized stock-based compensation expense for PSUs and performance-based options for which vesting became probable upon achievement of performance-based targets that occurred during their respective periods. As of December 31, 2019, the Company had an aggregate of $52,006 of unrecognized stock-based compensation cost, which is expected to be recognized over a weighted average period of 2.8 years.

Net Income Per Share

Net Income Per Share	3 Months Ended
	Dec. 31, 2019
Earnings Per Share [Abstract]
Net Income Per Share	9. Net Income Per Share Basic and diluted net income per share attributable to common stockholders was calculated as follows for the three months ended December 31, 2019 and 2018: Three Months Ended December 31, 2019 2018 (in thousands, except per share data) Basic net income per share: Numerator: Net income $ 13,443 $ 26,011 Denominator: Weighted average common shares outstanding—basic 19,751 19,426 Net income per share common share—basic $ 0.68 $ 1.34 Diluted net income per share: Numerator: Net income $ 13,443 $ 26,011 Denominator: Weighted average common shares outstanding—basic 19,751 19,426 Dilutive effect of common stock equivalents 1,022 1,384 Weighted average common shares outstanding—diluted 20,773 20,810 Net income per share common share—diluted $ 0.65 $ 1.25 Anti-dilutive common stock equivalents excluded from above 1,896 606

Income Taxes

Income Taxes	3 Months Ended
	Dec. 31, 2019
Income Tax Disclosure [Abstract]
Income Taxes	10. Income Taxes The Company’s provision for income taxes consists of federal and state taxes necessary to align the Company’s year-to-date tax provision with the annual effective rate that it expects to achieve for the full year. At each interim period, the Company updates its estimate of the annual effective tax rate and records cumulative adjustments as necessary. The authoritative guidance for accounting for income taxes allows use of the year-to-date effective tax rate (the “discrete method”) when a reliable estimate of the estimated annual effective tax rate cannot be made. During the three months ended December 31, 2019, the Company determined the use of the discrete method is more appropriate than the annual effective tax rate method due to the sensitivity of the Company’s tax rate to small changes in projected pre-tax earnings, which resulted in significant variations in the customary relationship between income tax expense and pretax income. As such, the Company has discretely calculated the tax provision based on pre-tax results through the three months ended December 31, 2019. For the three months ended December 31, 2019 and 2018, the Company recorded income tax expense of $1,504 and $3,730, respectively, both of which were attributable to the Company’s domestic operations. The Company has no foreign operations and therefore, has not provided for any foreign taxes. The Company files tax returns as prescribed by the tax laws of the jurisdictions in which it operates. In the normal course of business, the Company is subject to examination by federal and state jurisdictions, where applicable. The Company’s tax years are still open under statute from 2015 to the present. Earlier years may be examined to the extent that tax credit or net operating loss carryforwards are used in future periods. During 2018, the Company received notice of examination by the Internal Revenue Service (“IRS”) for the year ending September 30, 2016. The Company received and agreed to a notice of proposed adjustment from the IRS, which was paid in September 2018, the amount of which was immaterial to the financial statements. The Company is in the process of finalizing the completion of the IRS audit. During October 2018, the Company received notice of examination by the Massachusetts Department of Revenue (“DOR”) for the years ending September 30, 2015 and September 30, 2016. The Company received a notice of proposed adjustment from the DOR, the amount of which was immaterial to the financial statements. The Company is in the process of finalizing the completion of the DOR audit. The Company has not received notice of examination by any other jurisdictions for any other tax year open under statute. The Company had an unrecognized tax benefit of $1,679 and $1,650 as of December 31, 2019 and September 30, 2019, respectively. Unrecognized tax benefits represent tax positions for which reserves have been established. The Company’s policy is to record interest and penalties related to uncertain tax positions as part of its income tax provision.

Leases

Leases	3 Months Ended
	Dec. 31, 2019
Leases [Abstract]
Leases	11. Leases The Company has two real estate leases for properties located in Watertown, Massachusetts. The first lease, for office and laboratory space at 500 Arsenal Street, was effective in fiscal 2011 and expires in September 2022 with an option to extend the lease term for an additional five years. The second lease, for office space located at 400 Talcott Avenue, was effective September 2018 and expires in August 2024 with two options to extend the lease term for an additional three years each. The options to extend the lease terms were not included in the right-of-use assets and lease liabilities as they were not reasonably certain of being exercised. he Company’s office and laboratory space leases require the Company to pay for certain operating expenses based on actual costs incurred and therefore as the amounts are variable in nature are expensed in the period incurred and included in variable lease costs. In October 2019, the Company entered into an agreement to lease units of equipment over eighteen-month lease periods commencing upon shipment of each unit. The lease agreement contains an option to terminate the lease early, however this early-termination option was not included in the right-of-use asset and lease liability as it was not reasonably certain of being exercised. and therefore as these costs are variable in nature, they are expensed in the period incurred and included in variable lease costs. The components of lease expense for the Company’s real estate and equipment leases were as follows: Three Months Ended December 31, 2019 (in thousands) Operating lease cost $ 663 Short-term lease cost — Variable lease cost 479 $ 1,142 Other information: Three months ended December 31, 2019 (in thousands) Cash paid for amounts included in the measurement of operating lease liabilities $ 736 Operating lease liabilities arising from obtaining right-of-use assets $ 1,131 December 31, 2019 (in thousands) Weighted-average remaining lease term - operating leases (in years) 2.99 Weighted-average discount rate - operating leases 6.50 % Incremental borrowing rates were used to calculate the present value of the Company’s real estate and equipment leases. Future annual minimum lease payments under the Company’s real estate and equipment operating leases as of December 31, 2019 were as follows: Years ended September 30, (in thousands) 2020 $ 2,713 2021 3,457 2022 2,786 2023 608 2024 519 Total future minimum lease payments 10,083 Less: imputed interest (964 ) Total operating lease liabilities $ 9,119 Included in the balance sheet: December 31, 2019 (in thousands) Current operating lease liabilities $ 3,132 Operating lease liabilities, net of current portion 5,987 Total operating lease liabilities $ 9,119 As previously disclosed in the Company’s 2019 Annual Report on Form 10-K and under the previous lease accounting standard, ASC 840, Leases, the following table summarizes the future minimum lease payments due under the operating leases as of September 30, 2019: Years Ended September 30, Operating Leases Capital Leases (in thousands) 2020 $ 2,728 $ 93 2021 2,803 101 2022 2,684 99 2023 608 — 2024 519 — Thereafter — — Total $ 9,342 $ 293 In connection with one of the real estate leases, the Company has a total outstanding letter of credit in the amount of $608 $608

Commitments and Contingencies

Commitments and Contingencies	3 Months Ended
	Dec. 31, 2019
Commitments And Contingencies Disclosure [Abstract]
Commitments and Contingencies	12. Commitments and Contingencies Litigation and Contingencies Related to Use of Intellectual Property From time to time, the Company may become subject to legal proceedings, claims and litigation arising in the ordinary course of business. The Company currently is not a party to any threatened or pending litigation. However, third parties might allege that the Company or its collaborators are infringing their patent rights or that the Company is otherwise violating their intellectual property rights. Such third parties may resort to litigation against the Company or its collaborators, which the Company has agreed to indemnify. With respect to some of these patents, the Company expects that it will be required to obtain licenses and could be required to pay license fees or royalties, or both. These licenses may not be available on acceptable terms, or at all. A costly license, or inability to obtain a necessary license, would have a material adverse effect on the Company’s financial condition, results of operations or cash flows. The Company accrues contingent liabilities when it is probable that future expenditures will be made and such expenditures can be reasonably estimated. Indemnification Agreements In the ordinary course of business, the Company may provide indemnifications of varying scope and terms to customers, vendors, lessors, business partners, and other parties with respect to certain matters including, but not limited to, losses arising out of breach of such agreements or from services to be provided to the Company, or from intellectual property infringement claims made by third parties. In addition, the Company has entered into indemnification agreements with members of its board of directors and its executive officers that will require the Company, among other things, to indemnify them against certain liabilities that may arise by reason of their status or service as directors or officers. The maximum potential amount of future payments the Company could be required to make under these indemnification agreements is, in many cases, unlimited. To date, the Company has not incurred any material costs as a result of such indemnifications. In addition, the Company maintains officers and directors insurance coverage. The Company does not believe that the outcome of any claims under indemnification arrangements will have a material effect on its financial position, results of operations or cash flows, and it has not accrued any liabilities related to such obligations in its financial statements as of December 31, 2019.

Summary of Significant Accoun_2

Summary of Significant Accounting Policies (Policies)	3 Months Ended
	Dec. 31, 2019
Accounting Policies [Abstract]
Use of Estimates	Use of Estimates The preparation of consolidated financial statements in conformity with GAAP requires management to make estimates and assumptions that affect the reported amounts of assets and liabilities, the disclosure of contingent assets and liabilities at the date of the consolidated financial statements, and the reported amounts of revenues and expenses during the reporting period. Significant estimates and assumptions reflected in these consolidated financial statements include, but are not limited to, management’s judgments with respect to its revenue arrangements; valuation of Series 1 nonconvertible preferred stock and stock-based awards; the accrual of research and development expenses, and the accounting for income taxes, including uncertain tax positions and the valuation of net deferred tax assets. Estimates are periodically reviewed in light of changes in circumstances, facts and experience. Actual results could differ from the Company’s estimates.
Recently Adopted and Issued Accounting Pronouncements	Recently Adopted Accounting Pronouncements The Company adopted ASU No. 2016-02, Leases (Topic 842), as of October 1, 2019, using the modified retrospective method under ASU No. 2018-11, Leases (Topic 842): Targeted Improvements. The transition method allows entities to apply the transition requirements at the effective date rather than at the beginning of the earliest comparative period presented. The Company’s reporting for comparative periods is presented in accordance with ASC 840, Leases. Adoption of the new standard resulted in the recording of right of use (“ROU”) assets and lease liabilities of $7,151 and $8,622, respectively. The adoption of the standard did not have a material impact on the Company’s results of operations or cash flows. The Company elected to use the transition package of three practical expedients, which among other things, allowed the Company to carry forward the historical lease classification. The Company has elected, under Topic 842, the further practical expedient not to separate non-lease components from the lease components to which they relate and instead to combine them and account for them as a single lease component. The Company also elected the accounting policy election to keep leases with a term of twelve months or less off the balance sheet and to recognize payments for those leases on a straight-line basis over the lease term. ROU assets represent the Company’s right to use an underlying asset for the lease term, and lease liabilities represent the Company’s obligation to make lease payments arising from the lease. At the inception of the arrangement, the Company determines if an arrangement is a lease based on assessment of the terms and conditions of the contract. Operating lease ROU assets and lease liabilities are recognized at commencement date, and thereafter if modified, based on the present value of lease payments over the lease term. The lease term includes any renewal or early-termination options that the Company is reasonably assured to exercise. The present value of lease payments is determined by using the interest rate implicit in the lease, if that rate is readily determinable; otherwise, the Company uses its estimated secured incremental borrowing rate for that lease term. The underlying assets of the Company’s leases as of the adoption date consisted of office and laboratory space. In March 2017, the FASB issued ASU No. 2017-08, Receivables—Nonrefundable Fees and Other Costs (Subtopic 310-20): Premium Amortization on Purchased Callable Debt Securities (“ASU 2017-08”) which requires companies to amend the amortization period for premiums on debt securities with explicit call features to be the period through the earliest call date rather than through the contractual life of the debt instrument. This amendment aims to more closely align the recognition of interest income with the manner in which market participants price such instruments. The Company adopted the new standard on the effective date of October 1, 2019. T financial position and results of operations. Recently Issued Accounting Pronouncements In June 2016, the FASB issued ASU No. 2016-13, Financial Instruments—Credit Losses (Topic 326) (“ASU 2016-13”), which introduces a new methodology for accounting for credit losses on financial instruments, including available-for-sale debt securities. The guidance establishes a new “expected loss model” that requires entities to estimate current expected credit losses on financial instruments by using all practical and relevant information. Any expected credit losses are to be reflected as allowances rather than reductions in the amortized cost of available-for-sale debt securities. This amendment is effective for the Company in the fiscal year beginning October 1, 2020. The Company is currently evaluating the potential impact that ASU 2016-13 may have on its financial position and results of operations. Other accounting standards that have been issued or proposed by the FASB or other standards-setting bodies that do not require adoption until a future date are not expected to have a material impact on the Company’s consolidated financial statements upon adoption.

Fair Value of Financial Asset_2

Fair Value of Financial Assets and Liabilities (Tables)	3 Months Ended
	Dec. 31, 2019
Fair Value Disclosures [Abstract]
Financial Assets and Liabilities that were Subject to Fair Value Measurement on Recurring Basis	The following tables present information about the Company’s financial assets and liabilities that were subject to fair value measurement on a recurring basis as of December 31, 2019 and September 30, 2019, and indicate the fair value hierarchy of the valuation inputs utilized to determine such fair value: Fair Value Measurements at December 31, 2019 Using: Level 1 Level 2 Level 3 Total (in thousands) Assets: Cash equivalents: Money market funds $ 6,065 $ — $ — $ 6,065 Commercial paper — 17,423 — 17,423 Marketable securities: U.S. Treasury notes 176,856 — — 176,856 Commercial paper — 77,269 — 77,269 Corporate bonds — 133,043 — 133,043 $ 182,921 $ 227,735 $ — $ 410,656 Liabilities: Series 1 nonconvertible preferred stock $ — $ — $ 1,628 $ 1,628 $ — $ — $ 1,628 $ 1,628 Fair Value Measurements at September 30, 2019 Using: Level 1 Level 2 Level 3 Total (in thousands) Assets: Cash equivalents: Money market funds $ 44,569 $ — $ — $ 44,569 Marketable securities: U.S. Treasury notes 170,515 — — 170,515 Corporate bonds — 111,837 — 111,837 Commercial paper — 66,667 — 66,667 $ 215,084 $ 178,504 $ — $ 393,588 Liabilities: Series 1 nonconvertible preferred stock $ — $ — $ 1,628 $ 1,628 $ — $ — $ 1,628 $ 1,628
Fair Value Measurements of the Company's Outstanding Series 1 Nonconvertible Preferred Stock	The recurring Level 3 fair value measurements of the Company’s outstanding Series 1 nonconvertible preferred stock using probability-weighted discounted cash flow include the following significant unobservable inputs: Range (Weighted Average) December 31, September 30, Unobservable Input 2019 2019 Probabilities of payout 0%-60% 0%-60% Discount rate 5.75% 6.00%
Rollforward of Aggregate Fair Values of Outstanding Series 1 Nonconvertible Preferred Stock	The following table provides a rollforward of the aggregate fair values of the Company’s outstanding Series 1 nonconvertible preferred stock for which fair value is determined by Level 3 inputs: Series 1 Nonconvertible Preferred Stock Balance, September 30, 2019 $ 1,628 Change in fair value of nonconvertible preferred stock — Balance, December 31, 2019 $ 1,628

Marketable Securities (Tables)

Marketable Securities (Tables)	3 Months Ended
	Dec. 31, 2019
Investments Debt And Equity Securities [Abstract]
Fair Value of Available-for-Sale Marketable Securities by Type of Security	As of December 31, 2019 and September 30, 2019, the fair value of available-for-sale marketable securities, by type of security, was as follows: December 31, 2019 Amortized Cost Gross Unrealized Gains Gross Unrealized Losses Fair Value (in thousands) U.S. Treasury notes $ 176,712 $ 144 $ — 176,856 Corporate bonds 132,928 127 (12 ) 133,043 Commercial Paper 77,269 — — 77,269 $ 386,909 $ 271 $ (12 ) $ 387,168 September 30, 2019 Amortized Cost Gross Unrealized Gains Gross Unrealized Losses Fair Value (in thousands) U.S. Treasury notes $ 170,519 $ 60 $ (64 ) $ 170,515 Corporate bonds 111,690 170 (23 ) 111,837 Commercial Paper 66,667 — — 66,667 $ 348,876 $ 230 $ (87 ) $ 349,019

Accrued Expenses and Other Lo_2

Accrued Expenses and Other Long-Term Liabilities (Tables)	3 Months Ended
	Dec. 31, 2019
Payables And Accruals [Abstract]
Accrued Expenses and Other Current Liabilities as well as Other Long-Term Liabilities	Accrued expenses and other current liabilities, as well as other long-term liabilities, consisted of the following as of December 31, 2019 and September 30, 2019: December 31, September 2019 2019 (in thousands) Accrued expenses: Accrued research and development expenses $ 3,877 $ 6,936 Accrued payroll and related expenses 1,628 3,894 Accrued clinical manufacturing 2,580 3,447 Accrued professional fees 921 759 Accrued other 405 884 $ 9,411 $ 15,920 Other long-term liabilities: Uncertain tax positions $ 1,679 $ 1,746 Accrued rent expense — 900 Capital lease obligation — 200 Asset retirement obligation 254 254 $ 1,933 $ 3,100

Stock-Based Awards (Tables)

Stock-Based Awards (Tables)	3 Months Ended
	Dec. 31, 2019
Summary of Stock Option Activity Including Performance Based Options	The following table summarizes stock option activity, including performance-based options, for the year-to-date period ending December 31, 2019: Shares Issuable Under Options Weighted Average Exercise Price Weighted Average Remaining Contractual Term Aggregate Intrinsic Value (in thousands) (in years) (in thousands) Outstanding as of September 30, 2019 2,967 $ 46.54 6.7 $ 57,336 Granted 577 63.32 Exercised (76 ) 30.16 Forfeited (23 ) 75.36 Outstanding as of December 31, 2019 3,445 $ 49.53 6.8 $ 58,637 Options vested and expected to vest as of December 31, 2019 3,445 $ 49.53 6.8 $ 58,637 Options exercisable as of December 31, 2019 2,002 $ 38.28 5.4 $ 51,630
Summary of Restricted Stock Unit Activity	The following table summarizes the restricted stock unit activity for the year-to-date period ending December 31, 2019: Restricted Stock Units Weighted Average Grant Date Fair Value (in thousands, except per share data) Unvested at September 30, 2019 95 $ 30.00 Granted — — Vested (48 ) 30.00 Cancelled (1 ) 30.00 Unvested at December 31, 2019 46 $ 30.00
Stock-Based Compensation Expense	During the three months ended December 31, 2019 and 2018, the Company recognized the following stock-based compensation expense: Three Months ended December 31, 2019 2018 (in thousands) Research and development $ 2,492 $ 2,274 General and administrative 2,606 3,569 $ 5,098 $ 5,843 Three Months ended December 31, 2019 2018 (in thousands) Stock options $ 4,335 $ 3,904 Performance stock units 243 1,206 rTSRUs 362 531 Restricted stock units 158 202 $ 5,098 $ 5,843
Performance Share Units and Relative Total Stockholder Return Units [Member]
Summary of PSUs and rTSRUs Activity	The following table summarizes PSU and rTSRU activity for the year-to-date period ending December 31, 2019: PSUs rTSRUs Shares Weighted Average Grant Date Fair Value Shares Weighted Average Grant Date Fair Value (in thousands, except per share data) Unvested at September 30, 2019 41 $ 73.02 41 $ 67.76 Granted 27 63.94 27 45.81 Vested — — — — Cancelled — — — — Unvested at December 31, 2019 68 $ 69.37 68 $ 58.94

Net Income Per Share (Tables)

Net Income Per Share (Tables)	3 Months Ended
	Dec. 31, 2019
Earnings Per Share [Abstract]
Basic and Diluted Net Income Per Share Attributable to Common Stockholders	Basic and diluted net income per share attributable to common stockholders was calculated as follows for the three months ended December 31, 2019 and 2018: Three Months Ended December 31, 2019 2018 (in thousands, except per share data) Basic net income per share: Numerator: Net income $ 13,443 $ 26,011 Denominator: Weighted average common shares outstanding—basic 19,751 19,426 Net income per share common share—basic $ 0.68 $ 1.34 Diluted net income per share: Numerator: Net income $ 13,443 $ 26,011 Denominator: Weighted average common shares outstanding—basic 19,751 19,426 Dilutive effect of common stock equivalents 1,022 1,384 Weighted average common shares outstanding—diluted 20,773 20,810 Net income per share common share—diluted $ 0.65 $ 1.25 Anti-dilutive common stock equivalents excluded from above 1,896 606

Leases (Tables)

Leases (Tables)	3 Months Ended
	Dec. 31, 2019
Leases [Abstract]
Components of Lease Expense for Real Estate and Equipment Leases and Other Information Related to Leases	The components of lease expense for the Company’s real estate and equipment leases were as follows: Three Months Ended December 31, 2019 (in thousands) Operating lease cost $ 663 Short-term lease cost — Variable lease cost 479 $ 1,142 Other information: Three months ended December 31, 2019 (in thousands) Cash paid for amounts included in the measurement of operating lease liabilities $ 736 Operating lease liabilities arising from obtaining right-of-use assets $ 1,131 December 31, 2019 (in thousands) Weighted-average remaining lease term - operating leases (in years) 2.99 Weighted-average discount rate - operating leases 6.50 %
Schedule of Future Annual Minimum Lease Payments	Future annual minimum lease payments under the Company’s real estate and equipment operating leases as of December 31, 2019 were as follows: Years ended September 30, (in thousands) 2020 $ 2,713 2021 3,457 2022 2,786 2023 608 2024 519 Total future minimum lease payments 10,083 Less: imputed interest (964 ) Total operating lease liabilities $ 9,119
Schedule of Lease Included in Balance Sheet	Included in the balance sheet: December 31, 2019 (in thousands) Current operating lease liabilities $ 3,132 Operating lease liabilities, net of current portion 5,987 Total operating lease liabilities $ 9,119
Future Minimum Lease Payments	As previously disclosed in the Company’s 2019 Annual Report on Form 10-K and under the previous lease accounting standard, ASC 840, Leases, the following table summarizes the future minimum lease payments due under the operating leases as of September 30, 2019: Years Ended September 30, Operating Leases Capital Leases (in thousands) 2020 $ 2,728 $ 93 2021 2,803 101 2022 2,684 99 2023 608 — 2024 519 — Thereafter — — Total $ 9,342 $ 293

Nature of the Business and Ba_2

Nature of the Business and Basis of Presentation - Additional Information (Detail)	3 Months Ended
	Dec. 31, 2019
Organization Consolidation And Presentation Of Financial Statements [Abstract]
Entity incorporated, in year	1995

Summary of Significant Accoun_3

Summary of Significant Accounting Policies - Additional Information (Detail) - USD ($) $ in Thousands	1 Months Ended
	Oct. 31, 2019	Dec. 31, 2019	Oct. 01, 2019
Accounting Policies [Abstract]
Lessee operating lease, option to extend	true
Lessee, operating lease, description	The Company has elected, under Topic 842, the further practical expedient not to separate non-lease components from the lease components to which they relate and instead to combine them and account for them as a single lease component. The Company also elected the accounting policy election to keep leases with a term of twelve months or less off the balance sheet and to recognize payments for those leases on a straight-line basis over the lease term.
Right-of-use asset		$ 7,762	$ 7,151
Lease liability		$ 9,119	$ 8,622

Fair Value of Financial Asset_3

Fair Value of Financial Assets and Liabilities - Financial Assets and Liabilities that were Subject to Fair Value Measurement on Recurring Basis (Detail) - USD ($) $ in Thousands	Dec. 31, 2019	Sep. 30, 2019
Assets:
Marketable securities	$ 387,168	$ 349,019
Assets, Fair Value Disclosure, Total	410,656	393,588
Liabilities:
Liabilities, Fair Value Disclosure, Total	1,628	1,628
Series 1 Nonconvertible Preferred Stock [Member]
Liabilities:
Liabilities	1,628	1,628
Level 1 [Member]
Assets:
Assets, Fair Value Disclosure, Total	182,921	215,084
Level 2 [Member]
Assets:
Assets, Fair Value Disclosure, Total	227,735	178,504
Level 3 [Member]
Liabilities:
Liabilities, Fair Value Disclosure, Total	1,628	1,628
Level 3 [Member] | Series 1 Nonconvertible Preferred Stock [Member]
Liabilities:
Liabilities	1,628	1,628
Money Market Funds [Member]
Assets:
Cash equivalents	6,065	44,569
Money Market Funds [Member] | Level 1 [Member]
Assets:
Cash equivalents	6,065	44,569
Corporate Bonds [Member]
Assets:
Marketable securities	133,043	111,837
Corporate Bonds [Member] | Level 2 [Member]
Assets:
Marketable securities	133,043	111,837
Commercial Paper [Member]
Assets:
Cash equivalents	17,423
Marketable securities	77,269	66,667
Commercial Paper [Member] | Level 2 [Member]
Assets:
Cash equivalents	17,423
Marketable securities	77,269	66,667
U.S. Treasury Notes [Member]
Assets:
Marketable securities	176,856	170,515
U.S. Treasury Notes [Member] | Level 1 [Member]
Assets:
Marketable securities	$ 176,856	$ 170,515

Fair Value of Financial Asset_4

Fair Value of Financial Assets and Liabilities - Additional Information (Detail) - USD ($)	3 Months Ended
	Dec. 31, 2019	Dec. 31, 2018
Fair Value Disclosures [Abstract]
Transfers between Level 1, Level 2 and Level 3	$ 0	$ 0

Fair Value of Financial Asset_5

Fair Value of Financial Assets and Liabilities - Fair Value Measurements of the Company's Outstanding Series 1 Nonconvertible Preferred Stock (Detail) - Level 3 [Member]	Dec. 31, 2019	Sep. 30, 2019
Discount Rate [Member]
Fair Value Inputs, Liabilities, Quantitative Information [Line Items]
Range (Weighted Average)	0.0575	0.0600
Minimum [Member] | Probabilities of Payout [Member]
Fair Value Inputs, Liabilities, Quantitative Information [Line Items]
Range (Weighted Average)	0	0
Maximum [Member] | Probabilities of Payout [Member]
Fair Value Inputs, Liabilities, Quantitative Information [Line Items]
Range (Weighted Average)	60	60

Fair Value of Financial Asset_6

Fair Value of Financial Assets and Liabilities - Rollforward of Aggregate Fair Values of Outstanding Series 1 Nonconvertible Preferred Stock (Detail) - Series 1 Nonconvertible Preferred Stock [Member] $ in Thousands	Dec. 31, 2019USD ($)
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Beginning Balance	$ 1,628
Ending Balance	$ 1,628

Marketable Securities - Fair Va

Marketable Securities - Fair Value of Available-for-Sale Marketable Securities by Type of Security (Detail) - USD ($) $ in Thousands	Dec. 31, 2019	Sep. 30, 2019
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	$ 386,909	$ 348,876
Gross Unrealized Gains	271	230
Gross Unrealized Losses	(12)	(87)
Fair Value	387,168	349,019
Commercial Paper [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	77,269	66,667
Fair Value	77,269	66,667
Corporate Bonds [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	132,928	111,690
Gross Unrealized Gains	127	170
Gross Unrealized Losses	(12)	(23)
Fair Value	133,043	111,837
U.S. Treasury Notes [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	176,712	170,519
Gross Unrealized Gains	144	60
Gross Unrealized Losses		(64)
Fair Value	$ 176,856	$ 170,515

Marketable Securities - Additio

Marketable Securities - Additional Information (Detail) - USD ($) $ in Thousands	3 Months Ended
	Dec. 31, 2019	Sep. 30, 2019
Schedule of Available-for-sale Securities [Line Items]
Long-term marketable securities	$ 40,941	$ 65,013
Maximum [Member] | Short Term Marketable Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Maturity period of the marketable securities	1 year
Long Term Marketable Securities [Member] | Maximum [Member] | Corporate Bonds and U.S. Treasury Notes [Member]
Schedule of Available-for-sale Securities [Line Items]
Maturity period of the marketable securities	3 years
Long Term Marketable Securities [Member] | Minimum [Member] | Corporate Bonds and U.S. Treasury Notes [Member]
Schedule of Available-for-sale Securities [Line Items]
Maturity period of the marketable securities	1 year

Accrued Expenses and Other Lo_3

Accrued Expenses and Other Long-Term Liabilities - Accrued Expenses and Other Current Liabilities as well as Other Long-Term Liabilities (Detail) - USD ($) $ in Thousands	Dec. 31, 2019	Sep. 30, 2019
Accrued expenses:
Accrued research and development expenses	$ 3,877	$ 6,936
Accrued payroll and related expenses	1,628	3,894
Accrued clinical manufacturing	2,580	3,447
Accrued professional fees	921	759
Accrued other	405	884
Accrued expenses	9,411	15,920
Other long-term liabilities:
Uncertain tax positions	1,679	1,746
Accrued rent expense		900
Capital lease obligation		200
Asset retirement obligation	254	254
Other long-term liabilities	$ 1,933	$ 3,100

AbbVie Collaboration - Addition

AbbVie Collaboration - Additional Information (Detail) - AbbVie [Member] $ in Thousands	3 Months Ended
	Dec. 31, 2019USD ($)
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Cash consideration received under collaboration from sale of preferred stock, research funding payments, milestone payments and royalties	$ 922,000
Collaboration agreement tiered royalty description	from ten percent up to twenty percent, or on a blended basis from ten percent up to the high teens, on the portion of AbbVie’s calendar year net sales

Series 1 Nonconvertible Prefe_2

Series 1 Nonconvertible Preferred Stock - Additional Information (Detail) - Series 1 Nonconvertible Preferred Stock [Member] shares in Thousands	Dec. 31, 2019shares
Class Of Stock [Line Items]
Preferred stock, shares issued	1,931
Preferred stock, shares outstanding	1,931

Stock-Based Awards - Summary of

Stock-Based Awards - Summary of Stock Option Activity Including Performance Based Options (Detail) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended	12 Months Ended
	Dec. 31, 2019	Sep. 30, 2019
Options
Outstanding as of beginning of period	2,967
Granted	577
Exercised	(76)
Forfeited	(23)
Outstanding as of end of period	3,445	2,967
Options vested and expected to vest as of end of period	3,445
Options exercisable as of end of period	2,002
Weighted Average Exercise Price
Outstanding	$ 46.54
Granted	63.32
Exercised	30.16
Forfeited	75.36
Outstanding	49.53	$ 46.54
Options vested and expected to vest as of end of period	49.53
Options exercisable as of end of period	$ 38.28
Weighted Average Remaining Contractual Term
Outstanding as of end of period	6 years 9 months 18 days	6 years 8 months 12 days
Options vested and expected to vest as of end period	6 years 9 months 18 days
Options exercisable as of end of period	5 years 4 months 24 days
Aggregate Intrinsic Value
Aggregate Intrinsic Value	$ 58,637	$ 57,336
Options exercisable as of end of period	51,630
Options vested and expected to vest as of December 31, 2019	$ 58,637

Stock-Based Awards - Additional

Stock-Based Awards - Additional Information (Detail) - USD ($) $ in Thousands	3 Months Ended
	Dec. 31, 2019	Dec. 31, 2016
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Aggregate of unrecognized stock-based compensation cost	$ 52,006
Weighted average recognition period	2 years 9 months 18 days
Restricted Stock Units [Member] | Tranche One [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vesting percentage		50.00%
Vesting period		3 years
Restricted Stock Units [Member] | Tranche Two [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vesting percentage		50.00%
Vesting period		4 years
Executive Officers [Member] | Relative Total Stockholder Return Units [Member] | Minimum [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of shares range percentage	0.00%
Executive Officers [Member] | Relative Total Stockholder Return Units [Member] | Maximum [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of shares range percentage	150.00%

Stock-Based Awards - Summary _2

Stock-Based Awards - Summary of PSUs and rTSRUs Activity (Detail) shares in Thousands	3 Months Ended
	Dec. 31, 2019$ / sharesshares
PSUs [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Unvested, beginning balance | shares	41
Granted | shares	27
Unvested, ending balance | shares	68
Weighted Average Grant Date Fair Value, Unvested beginning balance | $ / shares	$ 73.02
Weighted Average Grant Date Fair Value, Granted | $ / shares	63.94
Weighted Average Grant Date Fair Value, Unvested ending balance | $ / shares	$ 69.37
rTSRUs [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Unvested, beginning balance | shares	41
Granted | shares	27
Unvested, ending balance | shares	68
Weighted Average Grant Date Fair Value, Unvested beginning balance | $ / shares	$ 67.76
Weighted Average Grant Date Fair Value, Granted | $ / shares	45.81
Weighted Average Grant Date Fair Value, Unvested ending balance | $ / shares	$ 58.94

Stock-Based Awards - Summary _3

Stock-Based Awards - Summary of Restricted Stock Unit Activity (Detail) - Restricted Stock Units [Member] shares in Thousands	3 Months Ended
	Dec. 31, 2019$ / sharesshares
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Unvested, beginning balance | shares	95
Granted | shares	0
Vested | shares	(48)
Cancelled | shares	(1)
Unvested, ending balance | shares	46
Weighted Average Grant Date Fair Value, Unvested beginning balance | $ / shares	$ 30
Weighted Average Grant Date Fair Value, Granted | $ / shares	0
Weighted Average Grant Date Fair Value, Vested | $ / shares	30
Weighted Average Grant Date Fair Value, Cancelled | $ / shares	30
Weighted Average Grant Date Fair Value, Unvested ending balance | $ / shares	$ 30

Stock-Based Awards - Stock-Base

Stock-Based Awards - Stock-Based Compensation Expense (Detail) - USD ($) $ in Thousands	3 Months Ended
	Dec. 31, 2019	Dec. 31, 2018
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Stock-based compensation expense	$ 5,098	$ 5,843
Stock Options [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Stock-based compensation expense	4,335	3,904
Performance Stock Units [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Stock-based compensation expense	243	1,206
rTSRUs [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Stock-based compensation expense	362	531
Restricted Stock Units [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Stock-based compensation expense	158	202
Research and Development [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Stock-based compensation expense	2,492	2,274
General and Administrative [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Stock-based compensation expense	$ 2,606	$ 3,569

Net Income Per Share - Basic an

Net Income Per Share - Basic and Diluted Net Income Per Share Attributable to Common Stockholders (Detail) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended
	Dec. 31, 2019	Dec. 31, 2018
Basic net income per share:
Net income	$ 13,443	$ 26,011
Weighted average common shares outstanding—basic	19,751	19,426
Net income per share common share—basic	$ 0.68	$ 1.34
Diluted net income per share:
Net income	$ 13,443	$ 26,011
Weighted average common shares outstanding—basic	19,751	19,426
Dilutive effect of common stock equivalents	1,022	1,384
Weighted average common shares outstanding—diluted	20,773	20,810
Net income per share common share—diluted	$ 0.65	$ 1.25
Anti-dilutive common stock equivalents excluded from above	1,896	606

Income Taxes - Additional Infor

Income Taxes - Additional Information (Detail) - USD ($) $ in Thousands	3 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Sep. 30, 2019
Income Tax Disclosure [Abstract]
Income tax expense	$ 1,504	$ 3,730
Unrecognized tax benefits	$ 1,679		$ 1,650

Leases - Additional Information

Leases - Additional Information (Detail) $ in Thousands	3 Months Ended
	Dec. 31, 2019USD ($)Lease	Oct. 31, 2019	Sep. 30, 2019USD ($)
Lessee Lease Description [Line Items]
Number of leases | Lease	2
Lease periods commencing upon shipment		18 months
Outstanding letter of credit	$ 608		$ 608
Restricted cash	$ 608		$ 608
First Lease [Member] | 500 Arsenal Street, Watertown, Massachusetts [Member]
Lessee Lease Description [Line Items]
Office lease expiration period	2022-09
Lease option to extend	an option to extend the lease term for an additional five years.
Second Lease [Member] | 400 Talcott Avenue, Watertown, Massachusetts [Member]
Lessee Lease Description [Line Items]
Office lease expiration period	2024-08
Period of lease agreement	2018-09
Lease option to extend	two options to extend the lease term for an additional three years each.

Leases - Components of Lease Ex

Leases - Components of Lease Expense for Real Estate and Equipment Leases and Other Information Related to Leases (Detail) $ in Thousands	3 Months Ended
	Dec. 31, 2019USD ($)
Leases [Abstract]
Operating lease cost	$ 663
Variable lease cost	479
Lease, Cost	1,142
Cash paid for amounts included in the measurement of operating lease liabilities	736
Operating lease liabilities arising from obtaining right-of-use assets	$ 1,131
Weighted-average remaining lease term - operating leases (in years)	2 years 11 months 26 days
Weighted-average discount rate - operating leases	6.50%

Leases - Schedule of Future Ann

Leases - Schedule of Future Annual Minimum Lease Payments (Detail) - USD ($) $ in Thousands	Dec. 31, 2019	Oct. 01, 2019
Leases [Abstract]
2020	$ 2,713
2021	3,457
2022	2,786
2023	608
2024	519
Total future minimum lease payments	10,083
Less: imputed interest	(964)
Lease liability	$ 9,119	$ 8,622

Leases - Schedule of Lease Incl

Leases - Schedule of Lease Included in Balance Sheet (Detail) - USD ($) $ in Thousands	Dec. 31, 2019	Oct. 01, 2019
Leases [Abstract]
Operating lease liabilities	$ 3,132
Operating lease liabilities, net of current portion	5,987
Total operating lease liabilities	$ 9,119	$ 8,622

Leases - Future Minimum Lease P

Leases - Future Minimum Lease Payments (Detail) $ in Thousands	Sep. 30, 2019USD ($)
Leases [Abstract]
2020	$ 2,728
2021	2,803
2022	2,684
2023	608
2024	519
Total	9,342
2020	93
2021	101
2022	99
Total	$ 293